U.S. patent application number 12/141993 was filed with the patent office on 2009-12-24 for high dynamic range image sensor.
Invention is credited to Christopher Parks.
Application Number | 20090314928 12/141993 |
Document ID | / |
Family ID | 41056981 |
Filed Date | 2009-12-24 |
United States Patent
Application |
20090314928 |
Kind Code |
A1 |
Parks; Christopher |
December 24, 2009 |
HIGH DYNAMIC RANGE IMAGE SENSOR
Abstract
An image sensor includes a plurality of pixels, each pixel
includes a first photosensitive region that collects charge in
response to light and having a first sensitivity; a second
photosensitive region that collects charge in response to light and
having a second sensitivity that is lower than the sensitivity of
the first photosensitive region; and a polarizer spanning the
second photosensitive region.
Inventors: |
Parks; Christopher;
(Rochester, NY) |
Correspondence
Address: |
Peter P. Hernandez, Patent Legal Staff;Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Family ID: |
41056981 |
Appl. No.: |
12/141993 |
Filed: |
June 19, 2008 |
Current U.S.
Class: |
250/208.1 |
Current CPC
Class: |
H01L 27/14629 20130101;
H01L 27/14627 20130101 |
Class at
Publication: |
250/208.1 |
International
Class: |
H01L 27/146 20060101
H01L027/146 |
Claims
1. An image sensor comprising: a plurality of pixels, each pixel
comprising: a first photosensitive region that collects charge in
response to light and having a first sensitivity; a second
photosensitive region that collects charge in response to light and
having a second sensitivity that is lower than the sensitivity of
the first photosensitive region; and a polarizer spanning the
second photosensitive region.
2. The image sensor as claim 1 wherein the polarizer is a wire grid
polarizer.
3. The image sensor as in claim 1 wherein the polarizer is oriented
to block light that is polarized by surface reflections.
4. The image sensor as in claim 1 further comprising a microlens
positioned over the first photosensitive region and partially
covering the second photosensitive region.
5. The image sensor as in claim 1 further comprising a third
photosensitivity region having a sensitivity that is substantially
equal or equal to the first sensitivity, and a fourth
photosensitive region having a sensitivity equal or substantially
equal to the second sensitivity.
6. The image sensor as in claim 5 further comprising a second
polarizer spanning the fourth photosensitive region.
7. The image sensor as in claim 6 wherein the second polarizer is a
wire grid polarizer.
8. The image sensor as in claim 7 wherein the second polarizer is
oriented to block light that is polarized by surface
reflections.
9. The image sensor as in claim 6 farther comprising a microlens
positioned covering the first photosensitive region and partially
covering the second and fourth photosensitive regions.
10. The image sensor as in claim 9 further comprising a second
microlens positioned covering the third photosensitive region and
partially covering the fourth photosensitive region.
11. An image capture device comprising: an image sensor comprising:
a plurality of pixels, each pixel comprising: a first
photosensitive region that collects charge in response to light and
having a first sensitivity; a second photosensitive region that
collects charge in response to light and having a second
sensitivity that is lower than the sensitivity of the first
photosensitive region; and a polarizer spanning the second
photosensitive region.
12. The image capture device as claim 11 wherein the polarizer is a
wire grid polarizer.
13. The image capture device as in claim 11 wherein the polarizer
is oriented to block light that is polarized by surface
reflections.
14. The image capture device as in claim 11 further comprising a
microlens positioned over the first photosensitive region and
partially covering the second photosensitive region.
15. The image capture device as in claim 11 further comprising a
third photosensitivity region having a sensitivity that is
substantially equal or equal to the first sensitivity, and a fourth
photosensitive region having a sensitivity equal or substantially
equal to the second sensitivity.
16. The image capture device as in claim 15 further comprising a
second polarizer spanning the fourth photosensitive region.
17. The image capture device as in claim 16 wherein the second
polarizer is a wire grid polarizer.
18. The image capture device as in claim 17 wherein the second
polarizer is oriented to block light that is polarized by surface
reflections.
19. The image capture device as in claim 16 further comprising a
microlens positioned covering the first photosensitive region and
partially covering the second and fourth photosensitive
regions.
20. The image capture device as in claim 19 further comprising a
second microlens positioned covering the third photosensitive
region and partially covering the fourth photosensitive region.
Description
FIELD OF THE INVENTION
[0001] The present invention generally relates to image sensors
and, more particularly, to image sensors having two photosensitive
regions. A polarizer covers one of the regions so that the
saturation of the two portions is different to accommodate
different lighting conditions.
BACKGROUND OF THE INVENTION
[0002] Image sensors that are used for cameras in automobiles need
to have a dynamic range more than 5 times greater than standard
image sensors. This is because direct or reflected sunlight
obscures detail in shadows. To image detail in shadows and
highlight conditions, several different methods have been used. One
method, such as U.S. Pat. No. 3,919,587, uses pixels with
photodiodes having a non-linear photo-response. In another method,
U.S. Pat. No. 6,777,661 uses pixels having two photosensitive
regions with high light and low light photo-response. Finally, U.S.
Pat. No. 6,069,377 uses a timer to measure the time it takes to
reach saturation and reads out this time out along with the
measured charge.
[0003] The disadvantage of the prior art is that reflected
sunlight, especially from a wet road surface, can be so bright that
image sensors with more than 10 times normal dynamic range are
still not sufficient to image objects in reflected sunlight and
shadows.
[0004] FIG. 1 illustrates light being reflected from a road
surface. The electric field vector of the light incident upon the
road surface is randomly polarized. The light has approximately
equal electric fields that are parallel and perpendicular to the
road plane of incidence. After reflection, the light has strong
polarization with the electric field vector parallel to the plane
of incidence (the road surface). It is noted that sunglasses are
often designed with a polarizer to filter out light with the
electric field vector parallel to the road surface.
[0005] A camera could place a polarizer in front of the image
sensor, but this is undesirable because of the extra cost, and it
removes 50% of the light from low light shadows.
[0006] The present invention provides an image sensor capable of
selectively rejecting sunlight reflected from road surfaces while
allowing light from other objects to be imaged. The present
invention also integrates the polarizer with the image sensor.
SUMMARY OF THE INVENTION
[0007] According to one aspect of the present invention, the
invention includes an image sensor comprising a plurality of
pixels, each pixel comprising a first photosensitive region that
collects charge in response to light and having a first
sensitivity; a second photosensitive region that collects charge in
response to light and having a second sensitivity that is lower
than the sensitivity of the first photosensitive region; and a
polarizer spanning the second photosensitive region.
[0008] These and other aspects, objects, features and advantages of
the present invention will be more clearly understood and
appreciated from a review of the following detailed description of
the preferred embodiments and appended claims, and by reference to
the accompanying drawings.
ADVANTAGEOUS EFFECTS OF THE INVENTION
[0009] The present invention has the advantages of having an image
sensor that selectively rejects sunlight, an integrated polarizer
and minimizes cost.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is an illustration of reflected sunlight off a road
surface;
[0011] FIG. 2 depicts a pixel of the image sensor of the present
invention;
[0012] FIG. 3 is a graph illustrating saturation of each of the two
photosensitive regions of FIG. 2;
[0013] FIG. 4 is an alternative embodiment of the present
invention;
[0014] FIG. 5 is a cross-sectional drawing of FIG. 4;
[0015] FIG. 6 is a top view of an image sensor of the present
invention; and
[0016] FIG. 7 is an image capture system having the image sensor of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] FIG. 2 shows a pixel 100 of the present invention having two
photosensitive regions 110 and 120, each of which collect charge in
response to incident light. One photosensitive region 110 has high
quantum efficiency so it can detect light from dark shadows, and
one photosensitive region 120 has low quantum efficiency. If the
light is too bright and saturates the high quantum efficiency
photosensitive region 110, then the signal is measured from the low
sensitivity region 120.
[0018] FIG. 3 shows the photo-response of the high sensitivity
region 300 (photosensitive region 110) compared to the low
sensitivity region 310 (photosensitive region 120). It is noted
that curve 310 reaches saturation at a much higher light level than
curve 300.
[0019] Referring back to FIG. 2, the photosensitive region 120
includes an integrated wire grid polarizer 130 for extending the
dynamic range. The polarizer 130 prevents the extremely bright,
reflected road surface light from saturating the low sensitivity
region 120. Wires spaced apart less than one wavelength, attenuate
light having an electric field polarized parallel to the length of
the wire. Light having an electric field polarized perpendicular to
the wire pass through more easily. The wire grid polarizer 130 is
oriented in the horizontal direction (parallel or substantially
parallel to the road surface) to prevent the road surface
reflection from being detected by the low sensitivity region
120.
[0020] FIG. 4 shows a pixel 200 with an alternate embodiment. The
pixel 200 includes two distinct high sensitivity regions 210 and
two distinct low sensitivity regions 220, each of which regions 210
and 220 collect charge in response to incident light. A wire grid
polarizer 230 covers both low sensitivity regions 220 each having
equal or substantially equal sensitivity. The high sensitivity
regions 210 also have equal or substantially equal sensitivity. The
two regions 210 and 220 are positioned diagonal to each other to
reduce image artifacts if a vertical line is imaged upon the
pixel.
[0021] FIG. 5 shows a cross section through the pixel 200 of FIG.
4. A micro-lens 260 covers the high sensitivity region 210 and
partially covers each of the two low sensitivity regions 220. It is
noted for clarity that one of the low sensitivity regions 220 is
from an adjacent pixel. This directs most of the light 250 towards
the high sensitivity region 210 and allows a smaller fraction of
light 240 to reach the low sensitivity region 220. This is what
makes the low sensitivity region 220 have less sensitivity. In
addition, the wire grid polarizer 230 filters out light 240 that
has been polarized by reflection from a road surface. The
configuration of the microlens covering the high sensitivity region
and partially covering the low sensitivity region also applies to
the embodiment of FIG. 2. In this embodiment, as in the alternative
embodiment, one of the low sensitivity regions 120 is from an
adjacent pixel.
[0022] FIG. 6 is a top view of the image sensor 305 of the present
invention having a plurality of pixels 100 and/or 200. The pixels
100 and/or 200 are arranged in a two-dimensional array, but they
may also be arranged in a one-dimensional array (i.e., linear
array). The image sensor 305 may be either a charge-coupled device
image sensor or a CMOS image sensor, both of which are well known
in the art.
[0023] FIG. 7 is a block diagram of an imaging system that can be
used with the image sensor 305 of the present invention. Imaging
system 1200 includes digital camera phone 1202 and computing device
1204. Digital camera phone 1202 is an example of an image capture
device that can use an image sensor incorporating the present
invention. Other types of image capture devices can also be used
with the present invention, such as, for example, digital still
cameras and digital video camcorders.
[0024] Digital camera phone 1202 is a portable, handheld,
battery-operated device in an embodiment in accordance with the
invention. Digital camera phone 1202 produces digital images that
are stored in memory 1206, which can be, for example, an internal
Flash EPROM memory or a removable memory card. Other types of
digital image storage media, such as magnetic hard drives, magnetic
tape, or optical disks, can alternatively be used to implement
memory 1206.
[0025] Digital camera phone 1202 uses lens 1208 to focus light from
a scene (not shown) onto image sensor 305 of imaging integrated
circuit 1212. Image sensor 305 provides color image information
using the Bayer color filter pattern in an embodiment in accordance
with the invention. Image sensor 305 is controlled by timing
generator 1214, which also controls flash 1216 in order to
illuminate the scene when the ambient illumination is low.
[0026] The analog output signals output from the image sensor array
305 are amplified and converted to digital data by
analog-to-digital (A/D) converter circuit 1218. The digital data
are stored in buffer memory 1220 and subsequently processed by
digital processor 1222. Digital processor 1222 is controlled by the
firmware stored in firmware memory 1224, which can be flash EPROM
memory. Digital processor 1222 includes real-time clock 1226, which
keeps the date and time even when digital camera phone 1202 and
digital processor 1222 are in a low power state. The processed
digital image files are stored in memory 1206. Memory 1206 can also
store other types of data, such as, for example, music files (e.g.
MP3 files), ring tones, phone numbers, calendars, and to-do
lists.
[0027] In one embodiment in accordance with the invention, digital
camera phone 1202 captures still images. Digital processor 1222
performs color interpolation followed by color and tone correction,
in order to produce rendered sRGB image data. The rendered sRGB
image data are then compressed and stored as an image file in
memory 1206. By way of example only, the image data can be
compressed pursuant to the JPEG format, which uses the known "Exif"
image format. This format includes an Exif application segment that
stores particular image metadata using various TIFF tags. Separate
TIFF tags can be used, for example, to store the date and time the
picture was captured, the lens f/number and other camera settings,
and to store image captions.
[0028] Digital processor 1222 produces different image sizes that
are selected by the user in an embodiment in accordance with the
invention. One such size is the low-resolution "thumbnail" size
image. Generating thumbnail-size images is described in commonly
assigned U.S. Pat. No. 5,164,831, entitled "Electronic Still Camera
Providing Multi-Format Storage of Full and Reduced Resolution
Images" to Kuchta, et al. The thumbnail image is stored in RAM
memory 1228 and supplied to color display 1230, which can be, for
example, an active matrix LCD or organic light emitting diode
(OLED). Generating thumbnail size images allows the captured images
to be reviewed quickly on color display 1230.
[0029] In another embodiment in accordance with the invention,
digital camera phone 1202 also produces and stores video clips. A
video clip is produced by summing multiple pixels of image sensor
305 together (e.g. summing pixels of the same color within each 4
column.times.4 row area of the image sensor array 305) to create a
lower resolution video image frame. The video image frames are read
from image sensor array 305 at regular intervals, for example,
using a 15 frame per second readout rate.
[0030] Audio codec 1232 is connected to digital processor 1222 and
receives an audio signal from microphone (Mic) 1234. Audio codec
1232 also provides an audio signal to speaker 1236. These
components are used both for telephone conversations and to record
and playback an audio track, along with a video sequence or still
image.
[0031] Speaker 1236 is also used to inform the user of an incoming
phone call in an embodiment in accordance with the invention. This
can be done using a standard ring tone stored in firmware memory
1224, or by using a custom ring-tone downloaded from mobile phone
network 1238 and stored in memory 1206. In addition, a vibration
device (not shown) can be used to provide a silent (e.g.
non-audible) notification of an incoming phone call.
[0032] Digital processor 1222 is connected to wireless modem 1240,
which enables digital camera phone 1202 to transmit and receive
information via radio frequency (RF) channel 1242. Wireless modem
1240 communicates with mobile phone network 1238 using another RF
link (not shown), such as a 3GSM network. Mobile phone network 1238
communicates with photo service provider 1244, which stores digital
images uploaded from digital camera phone 1202. Other devices,
including computing device 1204, access these images via the
Internet 1246. Mobile phone network 1238 also connects to a
standard telephone network (not shown) in order to provide normal
telephone service in an embodiment in accordance with the
invention.
[0033] A graphical user interface (not shown) is displayed on color
display 1230 and controlled by user controls 1248. User controls
1248 include dedicated push buttons (e.g. a telephone keypad) to
dial a phone number, a control to set the mode (e.g. "phone" mode,
"calendar" mode" "camera" mode), a joystick controller that
includes 4-way control (up, down, left, right) and a push-button
center "OK" or "select" switch, in embodiments in accordance with
the invention.
[0034] Dock 1250 recharges the batteries (not shown) in digital
camera phone 1202. Dock 1250 connects digital camera phone 1202 to
computing device 1204 via dock interface 1252. Dock interface 1252
is implemented as wired interface, such as a USB interface, in an
embodiment in accordance with the invention. Alternatively, in
other embodiments in accordance with the invention, dock interface
1252 is implemented as a wireless interface, such as a Bluetooth or
an IEEE 802.11b wireless interface. Dock interface 1252 is used to
download images from memory 1206 to computing device 1204. Dock
interface 1252 is also used to transfer calendar information from
computing device 1204 to memory 1206 in digital camera phone
1202.
[0035] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention.
PARTS LIST
[0036] 100 pixel [0037] 110 photosensitive region [0038] 120
photosensitive region [0039] 130 polarizer [0040] 200 pixel [0041]
210 high sensitivity region [0042] 220 low sensitivity region
[0043] 230 polarizer [0044] 240 light [0045] 250 light [0046] 260
micro-lens [0047] 300 high sensitivity region [0048] 305 image
sensor [0049] 310 low sensitivity region [0050] 1200 imaging system
[0051] 1202 digital camera phone [0052] 1204 computing device
[0053] 1206 memory [0054] 1208 lens [0055] 1212 imaging integrated
circuit [0056] 1214 timing generator [0057] 1216 flash [0058] 1218
A/D converter circuit [0059] 1220 buffer memory [0060] 1222 digital
processor [0061] 1224 firmware memory [0062] 1226 clock [0063] 1228
RAM memory [0064] 1230 color display [0065] 1232 audio codec [0066]
1234 microphone [0067] 1236 speaker [0068] 1238 mobile phone
network [0069] 1240 wireless modem [0070] 1242 RF Channel [0071]
1244 photo service provider [0072] 1246 Internet [0073] 1248 user
controls [0074] 1250 dock [0075] 1252 dock interface
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